Control of emissions from motor vehicles is now an accepted and necessary automotive design consideration throughout the world. In the United States since about 1965 (and earlier in California) all motor vehicles sold have incorporated some form of emission control. Recently, more vehicles sold for use in countries other than the United States have also been designed to reduce pollutants. The speed with which engine design changes have taken place to satisfy pollution reduction requirements has been extraordinary. Not only have the engine and vehicle manufacturers had to engage in major expenditures for facilities, equipment and accelerated technical achievement, but similarly, the automotive service industry has been experiencing a major upheaval in the effort to provide continuing qualified engine malfunction diagnosis and maintenance capability.
Exhaust emission standards in the United States have become increasingly stringent. Concurrent with the emphasis on reducing emissions, the automotive service industry throughout the world was beginning to acquire exhaust analyzers for measurement of hydrocarbons (HC) and/or carbon monoxide (CO). Now, the majority of qualified automobile service centers, particularly in the United States, use HC/CO analyzers routinely as an important diagnostic aid and also to inspect or verify vehicle manufacturer specifications at idle. In a number of areas within the United States, legislation requires service garages to have an approved HC/CO analyzer. CO analyzers have also become a mandatory part of emission conrols programs in a number of countries other than the United States.
Service facilities have found emission analyzers a useful diagnostic/service device during routine service inspection. For example, the use of a CO analyzer for properly adjusting carburetor balance and air/fuel ratios is now standard practice. Hydrocarbon measurements as an indication of ignition problems, a malfunctioning exhaust or intake valve, etc., is also widely used as a quick method of screening vehicles for further diagnosis by conventional oscilloscope testers.
The stringent standards in the United States for 1975 have forced most automobile manufacturers to use catalytic converters on current production vehicles to provide adequate control of exhaust emissions of hydrocarbons and carbon monoxide. Unfortunately, the effective use of HC/CO exhaust gas analyzers as a diagnostic aid for vehicles equipped with catalytic converters is more complicated than for vehicles without converters. In fact, if the converter is working properly, engine diagnosis with HC/CO analyzers is extremely difficult unless the vehicle has an exhaust sampling port ahead of the converter. When the catalytic converter is functioning properly, it oxidizes essentially all of the HC and CO to CO.sub.2 and water vapor. Consequently, the concentrations in the exhaust are so low they cannot be measured with accuracy with existing "garage-type" instrumentation. The changes in raw exhaust concentrations of HC, for example, as a result of intermittent misfire or "lean roll", no longer appear in the converted exhaust gases and the conventional exhaust gas analyzer loses value as a diagnostic tool.
The detection of a lean roll condition is becoming of great concern not only to the service garage owner doing after sale service, but, to the automobile manufacturer and his dealers as well. The leaner the mixture at which the carburetor is set, the greater the economy when the engine is operating in the manner for which it was designed. A slight deviation, however, can put the engine in a lean roll condition which increases both gasoline consumption and the emission of pollutants at the exhaust. The phenomenon of lean roll can best be understood with reference to FIG. 1. In a typical engine 10 having cylinders 12, a common intake manifold 14 is connected from a carburetor 16 to the intake ports 18 of cylinders 12. Obviously, the distance from the two outside cylinders 12 is greater than the distance to the two inside cylinders 12. The variation in the distances that the gasoline/air mixture must travel through the manifold 14 to reach the various cylinders 12 causes a difference in the mixture at the various cylinders 12 even though produced by a common carburetor 16. The mixture at the inner cylinders 12 tends to be richer than the mixture at the outer cylinders 12. In order to ignite and burn properly in the cylinders 12 the gasoline to air ratio of the fuel mixture must be within certain high and low limits. As the mixture is made more lean (less gasoline to a fixed volume of air) a point will be reached where the mixtue will not ignite.
Since, as stated earlier, the mixture at the two outer cylinders 12 tends to be leaner, as the mixture at the carburetor 16 is adjusted leaner, the two outer cylinders 12 will reach a point where they cyclically begin to misfire. This is called the lean-roll condition. As the mixture received drops below the critical level, the outer cylinders 12 misfire. This condition can occur individually or simultaneously as the exact mixture in any cylinder 12 at any instant is a function of many factors including the mixture at the carburetor at that instant (which may vary), the temperature of the manifold 14, whether the cylinder 12 fired on the last power cycle, and the amount of dilutants retained from the exhaust cycle during the intake stroke. Lean mixture can, of course, occur at all the cylinders 12 but we are concerned here with the slow roll phenomenon which is more correlatable to the outside cylinders (those furthest from the carburetor) in any engine.
Excess O.sub.2 in the exhaust gas of vehicles with catalytic converters has recently been identified as a major cause of increased sulfate emissions. It is known, that sulfates in high enough concentrations are injurious to health, and standards for their control in vehicle exhaust are being prepared in the United States. The net effect of this action will have a bearing on the automotive service industry. O.sub.2 concentrations and the source of O.sub.2 in the exhaust gas (i.e., from secondary air pumps or through modulated air bleeds to the induction system) will become an important control and adjustment parameter in the design as well as in the proper servicing of vehicles in subsequent years. Primarily because of the sulfate issue, it is expected that automotive engineers will no longer be able to use air pumps to deliver more air than necessary for efficient catalytic conversion. The service mechanic will be required to make precise adjustments of air control devices based on exhaust concentrations of O.sub.2. Of course, the measurement of HC and CO will continue to be important tools as well, in the complete diagnosis of engines and emission control systems.
In addition to reactor or converter equipped engines, 1975 saw the introduction of stratified charge engines as effective approaches to reach the present stringent exhaust emission standards. Several manufacturers incorporated "lean-burn" (available oxygen in excess of that required for stoichiometry) during at least some engine operating modes. Especially from a drivability point of view, these engines will require critical air-fuel ratio adjustments.
In considering the environment wherein the present invention is employed, the term "reactor" is used in a generic sense. Actually, reactor equipped vehicles can include thermal reactors, catalytic converters, manifold reactors or combinations of these. In a broad sense, they are similar in that they all need oxygen in excess of that required for normal combustion in the engine to work effectively.
Thermal reactors are simply insulated post-combustion chambers located in the vehicle exhaust manifolds. They burn combustibles under relatively low pressure conditions. To function effectively, they must be kept hot. Efficient combustion requires accurate control of secondary air rates and usually the basic engine must be adjusted on the rich side to provide adequate fuel value. Most thermal reactors utilize a separate ignition source (spark plug) to initiate and promulgate combustion.
Catalytic converters also must operate at relatively high temperatures (450.degree.-700.degree. C) to accomplish efficient conversion of HC and CO and CO.sub.2 and H.sub.2 O. Through 1977 it is expected that all catalytic converter vehicles will utilize oxidizing catalysts (rather than reduction catalysts) which require free O.sub.2 to function. The catalyst, which may be either in the pellet or monolithic form, is usually a substrate of alumina or similar material coated with a small amount of platinum and/or palladium. The noble metal catalyzes an oxidation reaction in the presence of HC, CO and O.sub.2 to accomplish coversion to non-toxic products of complete combustion. The reactions are: EQU 2CO + O.sub.2 .fwdarw. 2CO.sub.2 ; and, 2H.sub.2 + O.sub.2 .fwdarw. 2H.sub.2 O.
if there is not enough O.sub.2 available, conversion efficiency suffers and, if there is an excess of O.sub.2 available, the catalyst assists in the conversion of gasoline sulfur to undesirable SO.sub.3 which ultimately forms H.sub.2 SO.sub.4 and other sulfates. The problem of the production of H.sub.2 SO.sub.4 by catalytic converter equipped automobiles is one of great concern presently under study.
Manifold reactors are similar in many respects to thermal reactors, except they do not utilize a separate ignition souce. Air is pumped into the exhaust valve port area and in combining with the high temperataure exhaust gases, oxidation continues to occur for some distance in the exhaust system. Of course, excessive air tends to squelch the combustion rates and reduce the conversion efficiency.
As the complexity of the engines has increased, so has the need for regular maintenance. In an effort to ensure regular maintenance, the U.S. Federal Government is requiring certain areas to establish periodic vehicle inspection programs. The primary objective of these programs is to bring ambient air pollution levels to federally specified levels. As the untuned vehicle is a major cause of pollution from mobile sources, these programs are designed to force a malfunctioning vehicle to obtain an engine tune-up or to remove the defective vehicle from the road. The use of a suitable HC/CO analyzer by both Government enforcement agencies and authorized service garages has been made mandatory in a large number of these inspection programs.
The current emphasis on fuel conservation in the United States has given further incentive for the expansion of these periodic inspection programs. However, as mentioned previously, the use of an HC/CO tester as a diagnostic tool on reactor equipped vehicles is now limited. The reactors, when operating properly, are so efficient that 90 to 95% of the engine's CO and HC are oxidized and are now emitted as CO.sub.2 and water vapor. The indicators the tune-up technician has learned to rely upon with pre-reactor vehicles are now no longer present. If the technician has access to the exhaust system ahead of the catalytic reactor, he can still measure these indicators. However, most 1975 automobiles do not have access ahead of the reactor. Such accesses may be added in the future, but they are difficult to reach and extremely hot to handle.
The design objective to burn all of the fuel within the combustion chambers has been achieved to an amazing degree, in modern engines. However, just 1% incomplete combustion results in about 200 ppm of unburned HC in exhaust and 10% incomplete combustion, or 10% intermittent misfire results in approximately 2000 ppm unburned HC. When inadequate quantities of air are available, products of incomplete combustion such as CO and H.sub.2 are formed and if excessive air is available, some O.sub.2 is found in the exhaust. Uneven fuel distribution to each cylinder and mixture ratio variations from cycle to cycle within the same cylinder also account for significant quantities of unburned products in the exhaust.
Controlling air-fuel ratios within very narrow tolerances for all operating modes has become increasingly important as emission standards have tightened. Fortunately, the optimum air-fule ratio for minimum emissions (except NO.sub.x) is about the same as needed for maximum fuel economy, approximately 15.0-16.0 pounds of air to one pound of fuel. Maintaining control of the air-fuel ratio and consequently optimizing combustion efficiency becomes imperative if vehicles are to meet the emission standards and simultaneously have good fuel economy. Precision measurement of combustion efficiency in some form, therefore, becomes imperative at the service level.
Therefore, it is an object of the present invention to provide improved engine exhaust apparatus capable of detecting quantities of oxygen in the exhaust gas as well as providing a means of detecting engine misfire or lean roll.